[media] smiapp: Initialise rval in smiapp_read_nvm()

rval was not properly initialised in smiapp_read_nvm(). Do that.

Signed-off-by: Sakari Ailus <sakari.ailus@maxwell.research.nokia.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>